In motor vehicles of the above outlined character, the condition of engagement, or the setting, of the torque transmitting system (e.g., a clutch) or the torque which can be transmitted by the torque transmitting system is regulated and/or selected by a control unit and an adjusting or setting member; for example, a clutch can be actuated by a hydraulic system with a master cylinder and a slave cylinder as well as with an electric motor for the initiation of an actuation of the clutch. Such motor vehicles are known, for example, from the published German patent application No. 40 11 850.
For numerous modes of actuation of the torque transmitting system, it is advantageous (a) to know the desired clutch torque which has been selected in dependency on the distance covered by the means for actuating or adjusting the clutch and/or (b) to be in a position to relatively accurately select the torque which can be transmitted by the torque transmitting system. For example, a disengaging bearing can be utilized to set the torque transmitting system between a fully disengaged position or condition and a fully engaged position or condition. A play or idling distance or path exists between the fully disengaged position of the clutch and a position of engagement, called the "engagement" point or "gripping" point, when the torque transmitting system begins to transmit torque. As a rule, a progressively increasing transmission of torque takes place, as a function of the actuating path or movement of the clutch actuating means, such as a bearing, starting from the engagement point and terminating when the clutch is fully engaged. The engagement point denotes a condition of engagement of the clutch when the clutch begins to transmit torque or, alternatively, the engagement point can be defined as a position.
The knowledge or ascertainment of the engagement point is of utmost importance because the engagement point of a torque transmitting system, such as a clutch, characterizes that position of engagement or that extent of movement of the clutch engaging/disengaging means at which the transmission of torque begins, e.g., as a result of frictional engagement. If the characteristic curve of the clutch is known, and if the engagement point is also known, one can be said to have basically a full knowledge of the entire clutch characteristic.
In the course of its useful life and/or during the interval of actual use, the operation of a torque transmitting system such as, for example, a friction clutch, which is operated by a control unit and an adjusting member for the clutch, is subject to fluctuations which can have a number of different reasons or causes. For example, one reason for a shifting of the engagement point which actually exists in the clutch can be the increasing wear upon the component parts of the clutch, which wear takes place during the useful life of the clutch. An example of this type of wear would be the wear upon the friction linings. The changes which might develop or arise under such circumstances, such as in response to wear, settling phenomena or other change-causing processes, are changes which develop over relatively long periods and are of long duration. In addition, there can also develop relatively short-lasting fluctuations involving parts of the torque transmitting system, and the average duration of such relatively short-lasting changes can be in the range of from one or more seconds up to one or more hours. An example of a cause of short-lasting changes is the heating of component parts and the resulting thermal expansion of such component parts; these changes, too, can cause a shift in the position of the engagement point of the torque transmitting system.